Update documentation comments.

lib/src/utils.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library crypto.utils;
 
-// Constants.
+/// A bitmask that limits an integer to 8 bits.
 const MASK_8 = 0xff;
+
+/// A bitmask that limits an integer to 32 bits.
 const MASK_32 = 0xffffffff;
+
+/// The number of bits in a byte.
 const BITS_PER_BYTE = 8;
+
+/// The number of bytes in a 32-bit word.
 const BYTES_PER_WORD = 4;
 
-// Helper methods.
+/// Adds [x] and [y] with 32-bit overflow semantics.
 int add32(x, y) => (x + y) & MASK_32;
 
-int roundUp(val, n) => (val + n - 1) & -n;
-
-// Helper functions used by more than one hasher.
-
-// Rotate left limiting to unsigned 32-bit values.
+/// Bitwise rotates [val] to the left by [shift], obeying 32-bit overflow
+/// semantics.
 int rotl32(int val, int shift) {
   var mod_shift = shift & 31;
   return ((val << mod_shift) & MASK_32) |
       ((val & MASK_32) >> (32 - mod_shift));
 }